The American rider is being taken care of at the Maurizio Bufalini Hospital in Cesena but the medic's report isn't that good.
He started his MotoGP career with Honda in 2003 immediately after winning the AMA Superbike championship.
Now riding for Red Bull Honda in the World Superbike series, Hayden was rushed to the hospital with head and chest injuries.
Nicky Hayden, the 35-year-old motorcycle racer nicknamed the Kentucky Kid, was training on the Rimini coastline in Italy on his bicycle when a Peugeot 206 convertible hit him.
Former MotoGP world champion Nicky Hayden is in a "extremely critical" condition in intensive care after a cycling accident in Italy.
"Nicky Hayden's clinical condition remains extremely critical", said a statement from the Maurizio Bufalini Hospital in Cesena. This incident took place in Rimini in Italy, this Wednesday afternoon, and the site of a vehicle after that collision, shows the horrific intensity of this crash. He was 13th in the points following the latest Superbike race at Imola.
Hayden was joined at his bedside on Thursday by his mother Rose and younger brother Tommy.
There was continued support for Hayden and his family on social media throughout Thursday.
In 2016, he returned to the Superbike World Championship with the Ten Kate Racing-run Honda squad, now known as the Red Bull Honda World Superbike Team. That was reported by Reuters.
His career high so far came in 2006 when he won his only MotoGP championship.
Michele Scarponi, the 2011 Giro d'Italia cycling champion, was killed last month after being hit by a van while training close to his home in Filottrano in eastern Italy.